The present invention relates to an improvement in a reflector used in a light source for a projector such as a liquid crystal projector or an overhead projector.
Heretofore, a light source composed of an arc tube and a reflector for reflecting and emitting light from the arc tube, has been used as a light source for a projector such as a liquid crystal projector or an overhead projection. As the arc tube, there has been in general used a short arc type metal halide lamp in which metal halide is charged in an arc tube in order to use light emission inherent to the metal, and which has a short distance between electrodes distance. Further, as the reflector, there has been used a reflector in which the inner wall surface of a heat-resistant glass material is coated thereover with a multilayer film made of titanium oxide or silicon dioxide. These days, instead of the metal halide lamp, there have been prosperously used an extra high pressure mercury lamp which can easily exhibit a high intensity and a Xenon lamp which can exhibit a high glossy color property. Of these lamp, the extra high pressure mercury lamp has been improved in its light emitting efficiency by increasing the vapor pressure of mercury in the lamp up to a value higher than 120 atm during turn-on thereof so as to materialize the high bright intensity. Further, in addition to the mercury, an additive is mingled so as to improve a spectral distribution characteristic, thereby the glossy color property can be enhanced.
However, since the above-mentioned mercury lamp has an optimum operating temperature range which is narrow, there have been caused such problems that its luminous efficiency becomes lower or the use life of a lamp bulb thereof becomes shorter if it is used out of a desired optimum range.
A reflector used in the light source for a projector, is formed in a method comprising the steps of press-forming heat-resistant glass having a low thermal expansion rate, thereafter, coating the reflector over its inner surface with an aluminum vapor deposited film having a reflectance rate of 90%, and further, subjeting the aluminum vapor deposited film to an antioxidation process over its outer surface
These years, there has been increased such a market demand that the intensity of the lamp has to becomes higher, and accordingly, an optical multilayer film made of TiO2 of SiO2 has been prosperously used in order to obtain a reflectance rate higher than that of the aluminum vapor deposited film as the reflecting film on the inner surface of the reflector. A light beam projected from the reflector becomes in general a parallel light ray beam or a converged light ray beam. Accordingly, the shape of the reflecting surface of the reflector is in general parabolic or elliptic.
Referring to FIG. 1 which is a sectional view illustrating a light source for a projector used in general and using an extra-high pressure mercury lamp, an arc tube made of quartz glass, having a power consumption of 100 W class, and having an internal volume of 55 μl is enclosed therein with electrodes at its opposite ends with an arc length which is set in a range of about 1 to 4 mm. The arc tube 1 is charged therein with mercury as a luminescent substance, and argon as a start aid gas added with hydrogen bromide at a normal volume rate with respect to argon. An electrode wire rod 3 is welded thereto with molybdenum foils 4 so that, and lead wire electrode sealing parts 5 are formed. The electrode sealing part 5 on the reflector opening side is connected thereto with a lead wire fitting 19 as a power source terminal through the intermediary of a lead wire 18. A base 6 serving as another power source terminal is attached to the electrode sealing part 5 on the reflector bottom side. The base 6 is bonded and fixed to the bottom part of the reflector 7 formed over its inner surface with a multilayer reflection film for reflecting visual light and transmitting therethrough infrared rays, by means of cement 8. At this stage, the arc tube 1 is fixed so that the arc axis of the arc tube is located substantially at the focal point of the reflector. With the use of a flange part of the reflector at the front opening, a front face glass pane 9 having a thermal expansion rate the same as that of the reflector 7 is fitted. This front face glass pane 9 is adapted to prevent fragments of the arc tube from scattering when the arc tube bursts, and is applied at opposite surfaces thereof with anti-reflection coating.
FIG. 2 shows a use configuration in which an illumination optical system including the light source for a projector as shown in FIG. 1 is used as a light source for an optical instrument such as a liquid crystal projector, an overhead projector or the like. A cooling fan 10 is installed at a side or rear surface of the projector. Further, air from the cooling fan 10 is blown onto the reflector 7 so as to obtain a desired cooling effect. Alternatively, air around the light source which is heated by the light source which has been turned on is drawn out so as to create air stream in order to cool the reflector 7.
There has been used an image display element or a DMD (digital micro mirror device) such as a liquid crystal display panel, in which pixels are arranged in a matrix-like pattern, as measures for modulating the intensity of illumination light which has been uniformly distributed by the illumination optical system using the light source for a projector. TV signals or image signals from a computer are inputted to this image display element in order to display images on the screen thereof. The light from the light source is modulated by the image on the image display element. The modulated light is then magnified and projected through the intermediary of a projection lens. The so-called projection type image projector includes a separate screen on to which the magnified light is projected thereby. Meanwhile, the so-called rear-projection type image display apparatus includes a screen onto which the magnified image is projected on the rear side of the screen so as to display the image thereon. These image display apparatuses have been widely diffused at the market.